The title sets up a puzzle: someone who loves birds, and then a wolf eating a wing. The poem seems to ask whether a predator can feel sorry for what it does. "How sorry is the wolf" is half a question, half a quiet challenge, and the answer is left open.
The second stanza turns the wolf into something more abstract. It is "hard as the mind," as if the animal were a thought that won't soften or go away, something the mind has to "do something with." One way to read it is that the speaker, a bird lover, is the one troubled here. The wolf may stand for a cruel fact of nature, or for a harsh part of ourselves, and the speaker has to decide how to live with it.
The mood is spare, uneasy, and thoughtful. There is tenderness toward the bird, but no tidy comfort. The poem leaves us with a hard fact and no easy way to deal with it.